The present embodiments relates generally to water-based ink compositions, and in particular, invisible ink printable with an inkjet printing device. The present embodiments provide, in particular, a water-based invisible ink that can be detected by using a long wavelength ultraviolet (UV) light, for example, commonly used black light.
A printhead assembly of an inkjet printer typically includes one or more printheads each having a plurality of ink jets from which drops of liquid ink are ejected towards the recording medium. The ink jets of a printhead receive the liquid ink from an ink supply chamber, or manifold, in the printhead which, in turn, receives ink from a source, such as an ink cartridge. Each ink jet includes a channel having one end connected to the ink supply manifold. The other end of the ink channel has an orifice, or nozzle, for ejecting drops of ink. The nozzles of the ink jets may be formed in an aperture, or nozzle plate that has openings corresponding to the nozzles of the ink jets. During operation, drop ejecting signals activate actuators in the ink jets to expel drops of fluid from the ink jet nozzles onto the recording medium. By selectively activating the actuators of the ink jets to eject drops as the recording medium and/or printhead assembly are moved relative to each other, the deposited drops can be precisely patterned to form particular text and graphic images on the recording medium.
Recent applications of the above-described printing devices include use with invisible ink technology to print invisible messages on print substrates. For example, U.S. Patent Publication No. 2008/0220187, incorporated herein by reference in its entirety, discloses an invisible ink comprising invisible light-absorbing titania nanoparticles. Prints made with the described ink can be printed with high speed printheads and are invisible under normal light (e.g. 400-780 nm). However, such prints are also undetectable under long wavelength UV light (UV-A) (e.g., 365 nm). The prints can only be detected by exposing the printed area to short wavelength UV light (UV-B) such as, for example, 254 nm. However, light sources at such wavelengths are relatively rare and also present a safety hazard. As such, systems employing such inks are more costly and difficult to implement. For example, systems using the inks would require specialized close box detecting scanners to prevent exposure to the short wavelength UV light.
Therefore, the present embodiments provide a novel invisible ink composition that can be used with an inkjet printing device and which avoids the problems described above.